It has been known in the prior x-ray technology to move an x-ray tube circumferentially to successive angularly spaced points along an arc drawn from an isocenter which corresponds substantially to the center of the patient's body, transmitting a beam of x-rays toward the patient at each of the successive positions of the x-ray tube, and superimposing onto a common sheet of film a plurality of images resulting from the x-ray beams transmitted in the plurality of angularly spaced positions of the x-ray tube. The plurality of superimposed x-ray images obtained in this manner and superimposed upon each other on the same sheet of film results in a composite x-ray picture in which only one plane is in focus, namely, a plane passing through the isocenter. The technique just described is conventional x-ray tomography.
A more recent development in diagnostic x-ray technology has been the development of what is known as "computer assisted tomography." In x-ray diagnostic apparatus for use in computer assisted tomography, an x-ray tube, a collimator, a detector, and a data acquisition system are mounted for rotation as a unit interiorly of a gantry which houses the diagnostic imaging equipment. The gantry is provided with a hollow, centrally located, axially extending patient-receiving opening through which the patient may be moved linearly to predetermined desired positions relative to the x-ray apparatus. The x-ray tube and the collimator are mounted within the gantry on one side of the patient-receiving opening, and the detector and the data acquisition system are mounted within the gantry on an opposite side of the patient-receiving opening in diametrically opposed relation to the x-ray tube and collimator. The x-ray tube, collimator, and detector/data acquisition unit are all in fixed position relative to each other and define a diagnostic imaging assembly which rotates as a unit about the isocenter of the x-ray apparatus. As the diagnostic imaging assembly rotates through a single 360.degree. rotation at a predetermined constant speed, a multiplicity of x-ray views, such as 288 or 576 views, for example, are taken at a corresponding number of angular positions of the imaging assembly. This multiplicity of views defines the data which is digitized and sent to a computer where an image is reconstructed electronically.
The following publication is cited as a source of further background information on computed tomography:
"Introduction to Computed Tomography"--General Electric Company, 1976.
It is also possible to use the x-ray apparatus shown and described herein, including the collimator assembly of the invention, in static (non-rotating) computer-assisted radiographic procedures in which the diagnostic imaging assembly (comprising the x-ray tube, the collimator, the detector, and the data acquisition system) is not rotating.
In an x-ray apparatus for use in computed tomography, in order to maximize the image quality obtained, it is desirable that the radial distance between the focal point of the x-ray tube and the isocenter of the x-ray apparatus be reduced to a minimum, and that the detector which lies diametrically opposite from the x-ray tube and from the collimator should be positioned as far as possible radially outwardly from the outer periphery of the patient-receiving opening while still being confined within the hollow interior of the gantry.
It has also been found desirable to increase the diameter of the patient-receiving opening in the gantry as compared to the size of the patient-receiving opening in the prior art tomographic x-ray apparatus.
With the gantry having a larger diameter patient-receiving opening than in the prior art and with the x-ray tube and detector moved to optimum radial positions in the gantry relative to the isocenter of the apparatus, the radial space available for the collimator, which defines the diagnostic beam aperture, is reduced considerably, with the result that the collimator of the type used in the prior art computed tomography apparatus is too large in a radial direction to be received in the space available when the x-ray tube and the diametrically opposite detector device are positioned in their optimum radial positions for greatest operational efficiency and for optimum image resolution.